Lighting and effects. If you do it right, you can make 100 triangles look like 100,000.

You would need a million times the processing power to match the lighting and effects of Avatar.

Edit: I was bored so rather than my above guess, I calculated up the average power of the Avatar server farms, and checked on the average render time per frame of CGI, and it ends up being right around a million, so it was a pretty good guess.

Edit: Hmmm, I guess that would only be for one frame per second though, times by 30 or 60 for your prefered frame rate, 30-60 million times.